Asphaltenes are a class of complex mixtures in petroleum fluids, which are defined as the fraction in petroleum oils soluble in aromatic solvents, such as toluene, whereas insoluble in saturated hydrocarbons, such as n-heptane, and asphaltenes are prepared from crude oils by this simple principle. From this operational definition, asphaltenes are anticipated as a group of complex compounds, 1, 2 which are highly polydispersed and cannot be absolutely prescribed by some simplex physicochemical parameters. 3, 4 There are fewer large aggregates and narrower distributions once asphaltenes are in infinite-diluted systems of higher temperatures and better solvents; however, they still exhibit a real polydispersity. 1r4 The average molecular weight (MW) is not necessarily a good parameter to characterize asphaltenes, simply because asphaltenes are defined through their solubility in aliphatic hydrocarbons. 1 Time-resolved fluorescence depolarization (TRFD) was applied to determine the average MW and structural features of asphaltenes by the research group of Mullins, 5r12 and the authors arrived at three primary conclusions:(1) petroleum asphaltenes have an average molecular mass of∼ 750 Da (even smaller as 500 Da for coal-derived asphaltenes), with a full width at half-maximum of 500r1000 Da;(2) asphaltenes are dominated by molecules with one polycyclic aromatic hydrocarbon (PAH) unit per molecule; and (3) the most likely asphaltene PAH has roughly 7 rings. However, this method (TRFD) has been reported as being unsuitable for the study of asphaltenes. 2, 13, 14 The fundamental properties of asphaltenes, such as their MW are still in lively debate, 15r21 and the MW results obtained, even by the same method but from different laboratories, can be remarkably different from each other. 22r26

The equipment from Photon Technology International (PTI) used to obtain the TRFD results of asphaltenes are undoubtedly of high quality, as shown in the Mullins’ paper, 12 but it is still doubtful that TRFD analysis is necessarily suitable to study the fundamental properties of asphaltenes, such as their MWs and structural features, although the PTI has reported this kind of work on its website. 27 As anticipated, the TRFD results of asphaltenes indicated that the fluorescence emission wavelength has a broad distribution range from 380 to 650 nm and the rotational correlation time has a factor of 10 variations with the emission wavelength. 27 The authors further concluded that, because small asphaltene chromophores have a rotational correlation time 10 times smaller than that of the big ones, inside asphaltene molecules, the blue chromophores are not cross-linked to the red ones and then there is only one PAH unit per asphaltene molecule. 12